Transposon Tn1721: site-specific recombination generates deletions and inversions

Transposon Tn1721 contains genes for transposase (tnpA), resolvase (tnpR) and a resolution site (res). The closely linked loci were localized within a 3.8 kb region their order being res - tnpR - tnpA with res at the translational start of tnpR. Genes tnpR and tnpA have identical transcriptional polarity but independent promoters. The tnpR promoter had 40% of lac promoter efficiency its activity being autoregulated by binding of resolvase to res, as shown by fusion to the galactokinase gene. The weak tnpA promoter was only detectable in the transposase-mediated transposition reaction. Resolvase-catalyzed, site-specific recombination was analyzed in hybrid plasmids with either direct or inverted repeats of res. The rate of the reaction was dependent on the relative orientation of the two sites and on the provision of tnpR in cis or in trans. Direct repeats were rapidly resolved leading to deletions of intervening DNA, if the tnpR gene was provided in cis, but required approximately 60 generations for completion of the reaction, if tnpR was located on a second plasmid (in trans). The reaction involving inverted repeats of res (leading to inversion of intervening DNA) was only detectable if tnpR was furnished in cis. After 50 generations about 10% of plasmid DNA showed the inversion. The reaction was reversible.